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Q + Search at LEPS/SPring-8. Introduction Evidences for Q + Counter-evidences for Q + Preliminary results from new LEPS data Summary. T. Nakano ( RCNP, Osaka University ). Exotic Hadron WS @ NWU, May 27, 2005. What are penta-quarks?. Baryon. Minimum quark content is 5 quarks. - PowerPoint PPT Presentation
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+ Search at LEPS/SPring-8T. Nakano
( RCNP, Osaka University)
Exotic Hadron WS @ NWU, May 27, 2005
• Introduction• Evidences for +
• Counter-evidences for +
• Preliminary results from new LEPS data• Summary
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c.f.(1405): uudsu or uds
e.g. uuddc, uussd
What are penta-quarks? Baryon. Minimum quark content is 5 quarks. “Exotic” penta-quarks are those where the antiquark
has a different flavor than the other 4 quarks Quantum numbers cannot be defined by 3 quarks
alone.
Qqqqq
Example: uudds
Baryon number = 1/3 + 1/3 + 1/3 + 1/3 – 1/3 = 1
Strangeness = 0 + 0 + 0 + 0 + 1 = 1
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Prediction of the + Baryon
M1890-180*Y] MeV
D. Diakonov, V. Petrov, and M. Polyakov, Z. Phys. A 359 (1997) 305.
• Exotic: S=+1
• Low mass: 1530 MeV
• Narrow width: ~ 15 MeV
• Jp=1/2+
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Baryon masses in constituent quark model
• Mainly 3 quark baryons:M ~ 3Mq + (strangeness)+(symmetry)
• 5-quark baryons, naively:M ~ 5Mq + (strangeness) +(symmetry)1700~1900 MeV for +
mu ~ md = 300 ~ 350 MeV, ms=mu(d)+130~180 MeV
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•DPP predicted the with M=1530MeV, <15MeV, and Jp=1/2+.
•Naïve QM (and many Lattice calc.) gives M=1700~1900MeV with Jp=1/2-.
•But the negative parity state must have very wide width (~1 GeV) due to “fall apart” decay.
Theory
For pentaquark
Fall apart
Ordinary baryons
qq creation
•Positive parity requires P-state excitation.
•Expect state to get heavier.
•Need counter mechanism.
diquark-diquark, diquark-triquark, or strong interaction with “pion” cloud?
Positive Parity?
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Evidence for Penta-Quark StatesSpring8 ELSA
JLab-p
HERMES
ITEP
pp ++.
COSY-TOFDIANA
SVD/IHEP
JLab-d
ZEUSCERN/NA49
H1
Nomad
This is a lot of evidence
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Background level was estimated by using events from a LH2 target.
Assumption:• Background is from non-resonant K+K- production off the neutron/nucleus• … is nearly identical to non-resonant K+K- production off the proton
First evidence from LEPS nK+K-n
peak in events without a proton.
1.540.01 MeV< 25 MeV
Phys.Rev.Lett. 91 (2003) 012002
hep-ex/0301020
+
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Final state:
Ks + p
K+ + n
(Ks + p )
Mass
A few % difference from zero, but ~20% difference from the KN threshold.
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Width
• There is some inconsistency:– Most measurements are only upper limits.– DIANA has < 9 MeV. – The cross-section implies =0.9 MeV.– HERMES: = 13 +- 9 stat. (+- 3 sys.) MeV– ZEUS: = 8 +- 4 stat. (+- 5 sys.) MeV– Arndt et al. and Cahn et al. analysis of KN phase
shifts suggests that < 1 MeV !!
• The small width is the hardest feature for theorists to understand…
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• HERA-B (Germany):– reaction: p+A at 920 GeV– measured: K-p and K0p invariant mass– Clear peak for (1520), no peak for +
– production rate: +/(1520)<0.02• BES (China):
– reaction: e+e- J/ +-
– limit on B.R. of ~10-5
Negative Results
And many unpublished negative results
(HyperCP, CDF, E690, BaBar, LEP,,,).
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+ Search at CDF: pK0S Mass Spectrum+ Search at CDF: pK0
S Mass Spectrum
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Slope for mesons
Slope for baryons
Slope for pentaquarks??
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CLAS: New high statistics exp.Search for + in pK+Ksn
R. De Vita, APS April meeting, 2005
Limit on + production:Total cross-section < 1~4 nb
How could other low energy experiments with lower statistics could observe the signal?
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LEPS New LD2 and LH2 runs
• Data taken from Oct. 2002 to Jun. 2003.
• ~2 ・ 1012photons on a 15cm-long LD2 target.
•Less Fermi motion effect.
•LH2 data were taken in the same period with ~ 1.4 ・ 1012photons on the target.
#of photons: LH2:LD2 ≈ 2:3
we expect
# of events from protons: LH2:LD2≈ 2:3
# of events: LH2:LD2≈ 1:3
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Search for + in nK+K-n
MM (GeV) MM (GeV)
•A proton is a spectator (undetected).
• Fermi motion is corrected to get the missing mass spectra.
•Tight exclusion cut is essential.
•Background is estimated by mixed events.
preliminary
preliminary
L(1520)
pK+K-p nK+K-n
Can be consistent with the CLAS high stat result?
Next talk by S.I. Nam
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LEPS detector
1m
Good acceptance in the forward angle.
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New way to search for +
Θ + is identified by K - p missing mass from deuteron. ⇒ No Fermi correction is needed.
γ
p
n
Θ +
K -
p
γ
p
n
Θ +
K -
p
(1520)
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A possible reaction mechanism• + can be produced by re-scattering of K+.• K momentum spectrum is soft for forward going (1520).
γ
p/n
n/p
(1520)
K+/K0
PK obtained by missing momentum
Formation momentum
LD2
PK GeV/c
MM
d(γ
,K-
p)
GeV
/c2 LH2
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Final State Interaction
γ p
K+
K-
γp
K+
K-
p
p
n
nn
n
K-
K+
γ
pK+
pn
n
K-
p
γ
p
K+
K-
pn
n
K+
1520)
Large at low PpSmall, especially at low PK+
I.
II.
III.
Contributions from II and III are suppressed.
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Event selection
MMp(γ,K - p) GeV/c2 M(K - p) GeV/c2
Λ(1520)
Select Λ(1520) in 1.50–1.54 GeV/c2
⇒ calculate K- p missing mass of d K- p X reaction
K mass is smeared by Fermi motion. (assumed proton at rest)
LD2 data:
after selecting (1520)
inelastic events
select
LD2
γp→K - pKπ
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K - p missing mass in 1.50<M(K - p)<1.54 GeV/c2
MMd(γ,K - p) GeV/c2
1.53 GeV d (1520) X K- p
Good understanding of the background spectrum shape is crucial .preliminary
“You see the peak because you want it.
I see two dips.”
by Prof. I at KEK
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Major background process
• Quasi free (1520) production must be the major background.
• The effect can be estimated from the LH2 data
γ
p
n
(1520)
K+
n
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contribution
M(K - K+) GeV/c2
K+ momentum is estimated by missing momentum technique. Smeared by Fermi motion.
LD2 data
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K - p invariant mass after exclusion
M(K - p) GeV/c2 M(K - p) GeV/c2
E > 2.2 GeVall energy
LH2 LH2
non-resonant KKp events
non-resonant KKp events
Non-resonant KKp (phase space) contribution reproduces the spectrum shape under L(1520) well.
(1520)
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K - p missing mass for (1520) and non-resonant events in the
MMd(γ,K - p) GeV/c2 MMd(γ,K - p) GeV/c2
non-resonant KKp (1520)
Energy dependences are set to be the same.
No angular dependence has been introduced in the both reactions.
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K - p missing mass for (1520)
MMd(γ,K - p) GeV/c2 MMd(γ,K - p) GeV/c2
Quasi-free and non-resonant KKp
preliminary
preliminary
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K - p missing mass in the sideband regions
MMd(γ,K - p) GeV/c2 MMd(γ,K - p ) GeV/c2
1.46< M(K - p) < 1.50 GeV/c2 1.54< M(K - p) < 1.58 GeV/c2
: Energy and angular dependences were obtained by fitting to the data.
KKp: Enegy dependence fit to the data. Angulardependence flat.
LD2 LD2
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Test by side-band subtractionNarrow gate (1.52<M(K - p)<1.54 GeV/c2) = L + N Λ(1520) is enhanced.Wide gate (1.46<M(K - p)<1.58 GeV/c2) = L + 3N Non-resonant KKp is enhanced. ⇒ L = 1.5 (L+N) - 0.5 (L+3N) narrow gate wide gate
M(K - p) GeV/c2
L
NNN
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K - p missing mass for non-resonant KKp events (phase space distribution)
MMd(γ,K - p) GeV/c2 MMd(γ,K - p) GeV/c2
1.46< M(K - p) < 1.50 GeV/c2
1.50< M(K - p) < 1.54 GeV/c2
1.54< M(K - p) < 1.58 GeV/c2
sidebands averaged
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Side-band subtraction in KK invariant mass
M(K+,K-) GeV/c2 M(K+,K-) GeV/c2
after side-band subtraction
Since φ is not related with Λ(1520),φ peak disappears.
LD2
LD2
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K - p missing mass in(1520) and sideband region.
MMd(γ,K - p) GeV/c2 MMd(γ,K - p) GeV/c2
LD2 LH2
preliminary preliminary
1.50 < M(K - p) < 1.54 0.5(1.46 < M(K - p) < 1.50) +0.5(1.54 < M(K - p) < 1.58)
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Side-band subtracted K - p missing mass
MMd(γ,K - p) GeV/c2 MMd(γ,K - p) GeV/c2
Excess is related with Λ(1520).
Bump structure
LD2 LH2
preliminary
preliminary
Most events are below 1.52 GeV/c2
Events in MMd(γ,K - p) < 1.52 GeV/c2: LH2:LD2 = 1:1.3
# of photons: LH2:LD2 ≈ 2:3 (p) >> (n) for quasi free (1520) production.
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Energy dependence
MMd(γ,K - p) GeV/c2 MMd(γ,K - p) GeV/c2
LD2 LD2
preliminary preliminary
1.75 GeV < E < 2.1 GeV 2.1 GeV < E < 2.4 GeV(Emax)
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K - p missing mass in high and low energy region
MMd(γ,K - p) GeV/c2
preliminary
Below 2.1 GeV
Above 2.1 GeV
1.53 GeV Peak:•No change in the peak position. not likely due to kinematical reflections.•Narrower width in the low energy region. consistent with the detector resolution.
1.60 GeV Bump:•Only seen in the low energy region. threshold effects?•Not seen in LH2 data•Associated with (1520).
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contribution removed by angle cut
MMd(γ,K - p) GeV/c2 MMd(γ,K - p) GeV/c2
LD2 LH2
preliminary preliminary
Require cos Kp > 0
Excess remains.
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K - p missing mass in sideband regions
MMd(γ,K - p) GeV/c2
γ (1520)
K+/K0
γ p
K+/K0
K-
No peak!
+ formation cross-section by simple kaon re-scattering should be small.
LD2
A theoretical estimation by A. Titov is small.
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Width: Comparison with a MC spectrum
MMd(γ,K - p) GeV/c2
MC gives ~10 MeV resolution while Real data resolution is about ~ 8 MeV.
The statistical uncertainty of ~2 MeV is mainly due to fluctuations of the background at the tails of the peak.
preliminary
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Summary• Evidence for an S=+1 baryon around 1.54 GeV
with a narrow width has been observed by several experimental groups.
• There are some inconsistencies in the measured masses and widths.
• No signal has been observed in high energy experiments with high statistics and good mass resolution.
• If the + does exist, its production in high energy reactions must be highly suppressed.
• The + is not seen in the new CLAS high statistics data from a proton.
• If the + does exist, its production must have a large isospin asymmtery.
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Summary of LEPS new result• Peak is seen at ~1.53 GeV/c2 in the missing mass of the (,(1520)) reaction from a deuteron.• If is is real, the width seems to be very narrow.• The peak cannot be seen in the K-p invariant mass region outside of the (1520). • If the peak is due to the +, its production by re-scattering seems to be small in our kinematic region.• Enhancement (bump structure) around 1.6 GeV is also observed in the (,(1520)) reaction but only in the low E region.•Non-resonant KKp and contributions can be removed by sideband subtraction.•For quasi-free (1520) production, (p)>>(n).